The invention concerns a process for reducing the grain size of moist pre-crushed phosphate rock to obtain a grain size distribution in which 80 percent is smaller than 2 mm.
Phosphate rock is a hard, coarse-grained mineral. Among other things, it is used to produce phosphoric acid by reaction with sulfuric acid. For this reaction, it is necessary to reduce 80 percent of the phosphate rock to a grain size of under 2 mm.
Two processes have proved to be suitable for reducing the grain size of coarse-grained phosphate rock: dry crushing and wet crushing.
In dry crushing, the coarse-grained phosphate rock, which can have a moisture content of about 10 percent and more, is first subjected to drying whereby this moisture content is reduced to about 1-3 percent. This takes place in a large heated drying drum. Since all the phosphate rock is heated, even that which is finely granulated, high energy consumption for heating is inevitable. Approximately 50 kg of fuel oil per ton of P.sub.2 O.sub.5 are necessary to reduce then natural phosphate rock moisture of 14 percent to the residual moisture of 3 percent. The mills that grind the phosphate rock consume approximately 70 kWh per ton P.sub.2 O.sub.5. The screened material is separated into fine and oversize grain fractions by air classifiers.
The moisture content must be increased to over 50 percent for wet grinding in a ball or rod mill. The grinding process is impeded when the moisture content is under 50 percent, because the grinding parts and surfaces are encrusted and clogged.
The phosphate rock that is ground with a moisture content of over 50 percent must be dehydrated again for subsequent treatment. This dehydration is rather difficult since filter cloths with large pore diameters can only bring about an insufficient separation of the fine particles of phosphate rock from the liquid and, on the other hand, filter cloths with small pore diameters become clogged.
Although a combination of grinding and screening is indeed employed for reducing the size of crystals, only dry grinding of phosphate rock in combination with an air classifier has hitherto been commonly practiced, since none of the screens used was capable of properly separating coarse and fine particles due to the fact that the fine and coarse particles were caked together.
It must be said that it is quite possible to screen completely dry phosphate rock. However, with a moisture content between 1 and 5 percent, an electrostatic charge tends to build up resulting in the formation of clusters, the separation of the fine from the coarse grain then being very poor. Over half of the fines present in the starting material can be thus entrained by the rough grain.